Implantable medical devices (IMDs) can provide in situ physiological data monitoring and therapy delivery, including cardiac defibrillation, pacing and resynchronization, and intracorporeal drug delivery. IMDs function through pre-programmed control over monitoring and therapeutic functions. As required, IMDs can be interfaced to external devices, such as programmers, repeaters, and similar devices, which can program, troubleshoot, and exchange parametric and physiological data through induction or other forms of wireless telemetry.
Implantable sensors monitor and relay physiological data to other devices. Advanced conventional IMDs can be interfaced with implantable sensors directly through wired interconnections, or indirectly via an external intermediary device. Wired interconnections are invasive and require an intrabody tunnel that exposes a patient to adverse side effects, including internal injury, infection, and discomfort. Relay through an external intermediary device requires the periodic upload of stored data, which is only then available to an IMD indirectly by separate download. Thus, the physiological data can go stale before receipt by the IMD.
Alternatively, IMDs can be directly interconnected for data exchange through a wireless intrabody network, such as described in commonly-assigned U.S. Patent Publication No. 2006/0031378, published Feb. 9, 2006, pending, the disclosure of which is incorporated by reference. Multiple IMDs exchange data wirelessly through acoustic or radio frequency (RF) connections. Wireless intrabody interconnection alleviates the need for large onboard storage for holding monitored data, which can instead be immediately exchanged between the multiple IMDs.
Effective wireless intra-IMD communications, though, require maintaining interconnected IMDs in synchrony. Digital components within each device are susceptible to factors affecting performance, such as temperature, age, mechanical and electrical tolerances, and operational environment. Oscillators, for instance, control the timing of IMD operations, but frequency drift, which occurs naturally over time, can skew an oscillator's timing, which in turn skews IMD operation. Similarly, transducers convert electrical signals into and from acoustic energy. Channel characteristics can degrade transducer efficiency. Oscillator frequency drift can further degrade transducer efficiency by skewing transducer operation away from a trim frequency. Conventional IMD synchronization fails to compensate for oscillator-precipitated transducer skew.
U.S. Pat. No. 6,823,031, issued Nov. 23, 2004, to Tatem, Jr., discloses automated frequency compensation for remote synchronization. An oscillator is stepped through offset frequencies that are associated with a lock range of a phase lock loop (PLL). Oscillator frequencies are incremented until a desired result is achieved, after which control reverts back to the PLL. However, the oscillator for a device is trimmed without transducer adjustment.
U.S. Pat. No. 7,187,979, issued Mar. 6, 2007, to Haubrich et al., discloses medical device synchronization with reduced reliance on periodic polling. The internal clocks of two devices, such as an IMD and an external device, can be synchronized by computing time drift as a function of measured skew. In a further embodiment, an IMD is synchronized to a time signal generated by an external time reference with polling performed only when a device is likely to be receptive. However, oscillator synchronization is reliant on an external reference signal and is trimmed without transducer adjustment.
Therefore, an approach is needed to provide intracorporeal trim of transducers of wirelessly interconnected IMDs. Preferably, such an approach would avoid significant depletion of resources.